Frank Burke Porterfield's 1947 book, The Porterfields, documents the history of this Porterfield family. Chapter 10 of the book covers the Chester County, PA Porterfields. Here is how the chapter starts: "William Porterfield (1712-1789) m., 1734, Martha Alexander (1715-1789) of Chester County, Pennsylvania; apparently she came to Pennsylvania with her parents at the age of three years, and he before becoming of age, both were born in County Donegal, Ireland, of purely Scottish parentage."
Looks like you were right on their son Mathew. This same chapter of the book says he was never married, and contains his will ---- no children.
The Porterfields were Presbyterians. This branch of the Porterfields were only in Ireland for one generation.
Some more from the book: "Late in the seventeenth century, young William Porterfield left Kilmacolm, Scotland for Ireland. His older brother, Alexander, had inherited the family estate of Duchal when his grandfather John died. A generation later, William's son John left County Donegal, Ireland, with his wife and family for America. The Porterfield family had a long and colorful history in the area known as Renfrewshire, which is just west of Glasgow. This information is mainly taken from a small book by Elizabeth Main, A Kirk Without a Steeple, Being a History of the Old Kirk and Parish of Kilmacolm."